1. Field of the Invention
The present invention relates to the manufacture of soft, bulky nonwoven webs intended primarily for use in absorbent products. The manufacture of nonwoven webs has been accomplished by a variety of processes. Most of these involve the formation of fibers and filaments which are collected, usually in a more or less random array, and bonded. A wide variety of bonding steps has been developed depending upon the composition of the web, desired end use and other factors. These bonding steps include application of adhesives in a pattern or overall manner, and activation of adhesive fibers either chemically, for example, by solvent treatment, or by the use of heat and pressure to cause thermoplastic fibers to bond. Where the web contains thermoplastic fibers, widespread application has been made of the heat and pressure bonding process. In many cases, this provides high speed, low cost operation and, particularly when pressure is applied in a pattern, produces fabric-like properties. The present invention is directed to improvements in such processes and apparatus and products particularly adapted for uses requiring bulky, soft and very absorbent materials including thermoplastic fibers.
2. Description of the Prior Art
Thermoplastic fiber nonwoven webs are well known and described in a number of patents of which U.S. Pat. No. 4,041,203 to Brock and Meitner issued Aug. 9, 1977 is an example. It is also known to produce webs of mixtures of thermoplastic and nonthermoplastic fibers. Examples of such materials are disclosed in U.S. Pat. No. 4,307,721 to Tsuchiya and Mizutani dated Dec. 29, 1981; Canadian Pat. No. 1,012,420 to Marshall dated June 21, 1977; and U.S. Pat. No. 4,100,324 to Anderson, Sokolowski, and Ostermeier issued July 11, 1978. It is further known to use belts and wires for the formation of nonwoven webs, and U.S. Pat. No. 4,071,925 to Folk issued Feb. 7, 1978, U.S. Pat. No. 3,729,374 to Lissalde issued Apr. 24, 1973, U.S. Pat. No. 4,095,312 to Haley issued June 20, 1978 and U.S. Pat. No. 4,209,563 to Sisson dated June 24, 1980 are examples of teachings of such processes and apparatus. However, it remains desired to further improve such methods and apparatus, particularly as applicable to the production of bulky, soft, absorbent webs for applications in products such as sanitary napkins, disposable diapers, and the like. Existing methods and apparatus tend to result in overbonded webs reducing the effective absorbency and adversely affecting tactile and bulk properties.